The present invention generally relates to a safety control system such as for a nuclear power plant, and more particularly to a safety control system provided with two independent actuating means.
In nuclear power plants, it is a common practice to provide the plant with a safety and safeguard system for protecting the plant as well as a nuclear reactor against any possible abnormal transients and other unwanted phenomena for the purpose of assuring the safety of the nuclear reactor. By way of example, Japanese Laid-open Patent publication No. 118801/1986 (JP-A No. 61-118801) corresponding to U.S. Patent Application No. 666,696 filed Oct. 30, 1984 discloses a nuclear reactor safety and protection or safeguard system which includes sensors and channel signal processors connected in series, respectively, in a quadruple array and two logic circuits to which the outputs of the four channel signal processors are inputted. Each of the logic circuits is implemented in the form of two-out-of-four (2-out-of-4) voting logic circuit configuration, where one of the logic circuits is designed to produce a signal for activating a protecting system which can respond to the signal by opening a circuit breaker inserted in an electric power supply line leading to an electromagnetic device incorporated in a control rod controller unit to thereby scram the reactor, while the other logic circuit is designed to produce another safety system activating signal which brings about operation of an emergency borated water injection system and a spray system installed within a containment vessel of the reactor.
In connection with the 2-out-of-4 voting logic, typical examples thereof are found in a Japanese publication entitled "Nuclear Power Handbook", (1976), p.p. 263-267 and in particular on page 264, Table 9.6.
The aforementioned Japanese patent publication thus teaches the use of logic circuits implemented in the form of a 2-out-of-4 voting logic. However, no concrete circuit configuration of the 2-out-of-4 logic circuit is disclosed in this publication, although the abovementioned handbook shows in the Table 9.6 a typical example of the configuration of the 2-out-of-4 logic circuit.
More specifically, the Japanese Laid-open Patent Publication No. 118801/1986 concerns a safety control safegurd system for a pressurized water reactor (PWR). According to the safety control safeguard system for the reactor disclosed in this publication, one of the 2-out-of-4 logic circuits is utilized for activating the coil or solenoid incorporated in the control rod controller unit. In this conjunction, it is however noted that in the case of a control rod drive controller unit provided for assuring the safety of a boiling water reactor (BWR) known heretofore, the scramming electromagnetic valve for operating the controller unit is equipped with a pair of excitation coils. Consequently, according to the teachings disclosed in the Japanese patent publication mentioned above, the 2-out-of-4 logic circuit has to be provided for each of the excitation coils. For implementing the 2-out-of-4 logic circuit, the circuit configuration shown in the Table 9.6 on page 264 of the aforementioned handbook may be adopted.
Needless to say, when two independent manipulating or actuating means are provided for a single control system in concern (e.g. when two independent excitation coils are provided as in the case of the BWR), the two-out-of-four logic circuit has to be provided for each of the actuating means for activation thereof, which in turn means that the structure of the safety control safeguard system becomes very complicated, to disadvantage.